The Holy Avenger can be used by any character as a +2 sword with no special properties. Per the DMG, there are no side effects for any wielders unless the sword is intelligent in which case any non-lawful good character would take the normal damage based on the sword's ego every time they handled the sword.

Only a Paladin can use the Holy Avenger to its true potential

The full +5

Magic resistance of 50% in a 5' radius

Dispel Magic

+10 bonus damage against chaotic evil opponents

A Paladin is a subclass of Cavalier per the Unearthed Arcana. There is no mention of the Cavalier counting as a Paladin for usage of a Holy Avenger. Instead the Paladin gains several of the Cavalier's advantages:

In D&D 3.x and Pathfinder anyone can wield a Holy Avenger but only Paladins gain the complete benefits and use of the sword. For Pathfinder there is nothing to indicate that the Cavalier class (from the Advanced Player's Guide) would gain the paladin benefits when wielding a Holy Avenger.